The present invention relates to a color image processing apparatus for suitable use in a full color copying machine, and more particularly concerns a color image processing apparatus that includes a simplified conversion system for converting an image signal to a density signal.
There have been proposed color image processing apparatuses that optically read a character image, photographic image, or similar color images as divided into a red (R), green (G), and blue (B) colors, convert these colors to recording colors such as a yellow (Y), magenta (M), cyan (C), and black (K) colors, and record on recording paper on the basis of these colors using an output apparatus such as an electronically photographic color copying machine.
FIG. 33 is a block diagram of an example of the color image processing apparatuses. In the figure, number 1 indicates an R CCD which converts a red original image to red image signal, 2 is a G CCD which converts a green original image to green image signal, 3 is a B CCD which converts a blue original image to blue image signal.
The original image information (optical images) is color separated into red, green, and blue colors by a dichroic mirror (not shown), which are focused onto the R CCD 1, the G CCD 2, and the B CCD 3, respectively.
In the figure, number 4 indicates an A/D converter for converting the red image signal read by the R CCD 1 into eight bit digital data, 5 is an A/D converter for converting the green image signal read by the G CCD 2 into eight bit digital data, and 6 is an A/D converter for converting the blue image signal read by the B CCD 3 into eight bit digital data.
In the A/D conversions, also, shading corrections are made on the basis of pickup data of a reference white plate.
In the figure, number 7 indicates a standard density converter for converting the red, green, and blue eight bit digital image signals into six bit digital data, respectively. The digital image signals output of the standard density converter 7 are fed to a color code generator 9 to generate color codes. It should be noted that the standard density converter 7 is used only for generating the color codes.
The color codes are two bit codes to distinguish whether a pixel is white, black, or chromatic color. The two bit codes, for example, are of 00 for white, 11 for black, and 10 for color. Also, the digital image signals are fed to a multi-density conversion system. The multi-density conversion system of the example externally adjusts color balance and density, and selects either picture mode or character mode to adjust image quality.
The digital image signals, outputted from the A/D converters 4, 5, and 6, are first fed to a color balance control means 800. The color balance control means 800 corrects the density data corresponding with the red, green, and blue digital image signals to obtain necessary color balance on the basis of a control signal given from color balance key 102.
In turn, the digital image signals, after the color balance of those are adjusted, are fed to density control means 802. The density control means 802 corrects necessary density data of the red, green, and blue digital signals with a control signal given from a density control key 104.
In turn, mode conversion means 804 feeds out density correction data corresponding to the picture mode or character mode. Selection of any image quality mode is made with mode selection key 106 provided externally.
Through the above-mentioned density control stages are fed out six bit digital density signals. Number 10 indicates a color reproduction circuit that reproduces the colors, including the yellow (Y), magenta (M), cyan (C), and black (K), from the red, green, and blue signals depending on contents of the density signals, and feeds out six bit yellow (Y), magenta (M), cyan (C), black (K).
Number 29 indicates a color ghost corrector that corrects color ghost. This stage is needed against unnecessary color ghost produced around black characters. The color ghost correction is made in the way that a 1.times.7 window detects any color ghost, and the corrector converts to correct color code a pixel detected as color ghost. The color ghost correction is made in a main scanning direction and an auxiliary scanning direction. Color ghost corrector 29 can be achieved by way of the technique disclosed in the Japanese Patent Laid-Open 1-195775.
Number 30 indicates a marking color conversion circuit that detects areas marked on an original and converts the areas to a marking color. The circuit can feed out a density signal D and a marker area signal Q of the marking color.
Number 80 indicates an image processor that makes image processes such as filter process, magnification process, and shading. Number 82 indicates a PWM multivalue converter that converts the six bit density signal to multivalue in way of pulse width modulation (PWM). Number 84 indicates a printer unit that forms color images in the way that toner images Y, M, C, and K are put one over another on a sensitizing drum (OPC).
In the prior color image processing apparatus mentioned above, the density conversion system that converts the image signals to density signals is complicated in construction as it includes the color balance control means 800, the density control means 802, and mode conversion means 804 accordingly. This results in high cost of the arrangements.
In view of the foregoing, it is a first object of the present invention to provide a color image processing apparatus which includes a simplified density control system, thereby solving the problem mentioned above.
Also, in the prior color image processing apparatus mentioned above, as described above, the image information input from the original is usually converted to red, green, and blue signals. The colors recorded by the printer unit 84, however, are usually yellow, cyan, magenta, and black which are complements to those colors.
The scanner spectral sensitivity and toner spectral reflectance are different as shown in FIG. 34. Red, green, and blue density levels obtained on the basis of scanner levels, therefore, are converted to cyan, magenta, and yellow toner density levels by way of a linear masking method. The linear masking can be expressed as ##EQU1## where Dr, Dg, and Db denote the density levels to which the scanner red, green, and blue luminance levels are converted, respectively; Dc, Dm, and Dy are the density levels to which amounts of the cyan, magenta, and yellow toners are converted; and, aij (i, j=1, 2, 3) is a masking coefficient.
If the masking coefficient (matrix coefficients) all through a33 are obtained from three sample data C, M, and Y, as an example, the original colors, including cyan, magenta, and yellow, are virtually identical with the ones after copying as seen from a L*a*b* uniform color space system of coordinates in FIG. 35.
However, for the other colors, the conversion errors are marked, as the linear masking method itself is an approximate equation.
Even if the number of sample data is increased, it cannot be expected that the conversion errors will be effectively reduced, as the number of colors that are theoretically treated with the one linear masking matrix is limited to three.
This means that if the color reproduction process is made only by the one linear masking matrix, the color reproduction of a chromatic color is deteriorated in the printer section.
Also, the linear masking process is arranged so that achromatic colors such as black can be fed out as cyan, magenta, and yellow signals having such a density ratio that their equivalent achromatic color densities should be identical.
However, the density ratio of the cyan, magenta, and yellow signals to reproduce an achromatic color is different in a high density portion and a low density portion. If the density ratio of the achromatic color being effective at the low density portion is fed out through the linear masking process mentioned above, for example, it is hard to maintain gray balance at the high density portion. In other words, where there is such a density difference, reproduction of the achromatic color is deteriorated.
In view of the foregoing, it is a second object of the present invention to provide a color image processing apparatus which can improve the reproductibilities of both the chromatic and achromatic colors, thereby solving the problem mentioned above.